As known, the requirements posed to the gate and/or tunnel oxides are becoming stricter and stricter with the increase of the integration level; therefore the availability of methods for evaluating the oxide reliability is very important.
A main problem in nonvolatile EEPROM and flash-EEPROM devices resides in the difficulty of measuring the distribution of the threshold voltage of memory cells, both at the end of the manufacturing process (so as to obtain an index of the fluctuations in the manufacture) and after cycling (so as to have an index of the lack of uniformity of the memory device degrade).
In particular, a reliable method for precisely measuring the threshold voltage distribution is very useful during debugging and may become very important also for qualifying the new products.
The reliability of the tunnel oxide may be measured through very simple structures, such as capacitors, using different techniques (e.g., constant current stress or linear ramp voltage stress, exponential ramp current stress). The obtainable information is however not always indicative of the real structure, since the cell geometry is substantially different from the capacitor geometry.
Presently, the cell threshold may be obtained only when the device is operating, that is at a very late design and manufacture stage.
Furthermore, U.S. Pat. Nos. 5,515,318, 5,604,699, 5,712,816 and 5,793,675 disclose a test structure and method for measuring the tail of the threshold distribution at low voltage with high precision (so as to detect even a single defective cell) for flash-FEPROM devices. The test structure has all bit lines connected in parallel to a single drain pad (all the drains of the cells are connected to each other), all the word lines connected in parallel to a single control gate pad (all the control gates are connected to each other); furthermore also the sources are connected together. Thereby, the threshold of all the cells may be read in parallel; in particular, comparing the threshold distributions obtained before and after applying a stress, it is possible to point out the shift of the tail due to a single defective cell.
This structure does not allow the measurement of the whole distribution. Indeed, the increase of the control gate voltage causes an increase in the read current (which is the sum of the current of the single cells, which are gradually turning on); however the increase of the current is limited by the resistance in series to the circuit and by the distributed resistance of the circuit. Thus the structure becomes progressively less sensitive with the increase of the cells having a threshold voltage lower to a given control gate voltage.
Another limitation of the known structure is that it is not possible to program the entire array under the working conditions of the device. Programming an entire array of flash-EEPROM cells would indeed require a high channel current for generating hot electrons, and the above cited current limitation prevents the required current values to be reached.